1. Field of the Invention
The present invention relates to asphalt pavement anti-stripping additives, and more particularly to the use of polyphosphoric acid in conjunction with an alkaline additive to improve the adhesion between an asphalt binder and aggregate.
2. Description of the Related Technology
As is well known, asphalt is commonly used as a paving material. Typically, the asphalt, often referred to as “asphalt cement” or “asphalt binder,” is mixed with an aggregate to form an asphalt concrete suitable for paving. Thus, the asphalt concrete comprises aggregate held within a continuous phase of the asphalt binder by adherence of the asphalt binder to the aggregate. Unfortunately, however, asphalt binder has a tendency to lose its adhesive bond with the aggregate, particularly in the presence of moisture, in a process known as “stripping.” Specifically, the adhesion between polar molecules within the asphalt and polar molecules on the aggregate surface is disrupted by water (a polar molecule) from rain or underground sources. The stripping of asphalt binder from aggregate surfaces reduces the life of the pavement and is a serious problem throughout the many millions of miles of highways throughout the world. In addition to stripping, water acts like a solvent in asphalt thereby decreasing asphalt viscosity, reducing strength, and increasing rutting.
In view of the foregoing, numerous efforts have been made over the years to reduce asphalt stripping. Many such efforts have been directed to including various additives to the asphalt binder compositions or even to the aggregate to increase the binder-aggregate adhesion. A typical type of anti-stripping additive comprises surface-active agents such as amines, preferably liquids, that have polar head-groups that exhibit an affinity for polar surfaces like that of the aggregate. The amines also typically contain long, non-polar fatty chains that exhibit a high affinity for asphalt binder. The molecular structure of a surface-active amine also tends to lower the interfacial tension between the asphalt binder and the aggregate, thereby increasing the strength of the adhesive bond between the two. Examples of such polyamines include AD-HERE HP PLUS (a trade designation of Arr-Maz Custom Chemicals, Inc. of Winter Haven, Fla., USA) and PAVE-BOND LITE (a trade designation of Rohm and Haas). Such anti-strips are usually mixed with the asphalt binder prior to pumping the modified binder to the mixing plant.
Alternatively, the modification of asphalt binders with polyphosphoric acid has been known for quite some time (see, e.g., U.S. Pat. No. 3,751,278), although its benefits as an anti-stripping additive may not have been recognized until more recently. Polyphosphoric acids may be reacted with asphalt to increase the asphaltene fraction or asphaltene dispersion of the binder. This change is the believed to be the primary reason for a change in the temperature-viscosity relationships of the asphalt binder. Specifically, at elevated temperatures, a binder comprising polyphosphoric acid tends to have a higher viscosity than the same binder without polyphosphoric acid. Conversely, at lower temperatures, a binder comprising polyphosphoric acid tends to have a low viscosity that the same binder without polyphosphoric acid. A secondary reason for the increase of the high temperature viscosity is believed to be hydrogen bonding between un-reacted acid (free acid sites) and the asphalt. Some studies indicate that the hydrogen bonding may be at least partially neutralized or reversed by the presence of conventional amine anti-strips, and as a result, there is a belief by some of those skilled in the art that polyphosphoric acid and amine anti-strips should not be used together.
A further alternative for improving adhesion is the use of hydrated lime (often simply referred to as “lime”) in paving compositions. Typically, lime is added to the paving composition by treating the aggregate. The use of lime in asphalt paving has been known for at least 80 years, but its benefits as an anti-stripping additive was not known until more recently. Specifically, it is believed by those of skill in the art that lime reacts with highly polar molecules to form insoluble salts that tend not to attract water. This prevents said polar molecules from reacting with other molecules in the paving composition to form water-soluble soaps that promote stripping (see, e.g., Petersen, J. C., H. Plancher, and P. M. Harnsbergen, “Lime Treatment of Asphalt to Reduce Age Hardening and Improve Flow Properties,” Proceedings, AAPT, Vol. 56, 1987).
An additional method of improving adhesion by treating the aggregate includes applying polymer coatings to the particles (see, e.g., U.S. Pat. Nos. 5,219,901 and 6,093,494). Specifically, U.S. Pat. No. 5,219,901 discloses a technique for reducing stripping tendencies that involves coating the aggregate with a thin, continuous film of a water-insoluble high molecular weight organic polymer, such as an acrylic polymer or a styrene-acrylic polymer.
Although many of the foregoing methods of improving stripping resistance have been effective to various degrees depending on numerous factors such as the type of asphalt, type of aggregate, amount of additive, etc., a need continues to exist for an asphalt concrete or paving composition having increased adherence or anti-stripping behavior in addition to other qualities that make it a desirable paving material (e.g., cost, ease of use, resistance to rutting, cracking, fatigue, oxidation and aging, etc.).